Several suitable superelastic alloys are known for use in medical devices particularly Nickel Titanium (Ni-Ti) alloys. These alloys find many applications in medicine They are bio-compatible and exhibit shape-memory characteristics applicable for many purposes. Regarding the objectives of the present invention, the materials are characterized by their superelastic properties whereby, upon being mechanically loaded, a shape change up to about 8% at constant applied force is possible. Accordingly, these alloys may be used as force-transmitting components which may be substantially deformed without rupturing or which illustratively are used as force-limiting means in the manner described in the German patent document Cl 43 13 903.
However, certain manufacturing problems are obstacles when wishing to use the superelastic alloys on a wider scale for the purposes above. In the state of the art as known to-date, and as stated in the above patent document, the manufacture of superelastic alloys requires complex adjustment of the accurate phase state. Moreover, the alloy composition must be quite rigorous, and entails costly manufacturing. Conventional subsequent working is hardly feasible.
As regards the described pull rod 6 (FIG. 1) of the above cited document, problems arise in coupling the load-receiving components 5, 10 to the ends of said element. These coupling sites can be made in the state of the art only if the pull rod was fitted at the factory with prefabricated coupling sites and if they were thermally pretreated. The state of the art precludes using segments of spool-fed wire that would be highly economical.
If segments of superelastic alloy spool-fed wire were used, they could not be worked. Because of the material's superelastic properties, cold-forming or milling are impossible. Additionally, it is impossible to solder these alloys and achieve adequate soldering strength. Welding is excluded because of thermal stress.
All attempts for post-working superelastic alloys have remained unsuccessful or entail mechanical or thermal stressing of the alloy, destroying its superelastic properties and furthermore degrading the material strength. After such treatment, the material will break upon subsequent stressing.
Thus, in the state of the art, force-transmitting elements of superelastic materials must be manufactured in their final shape using the required procedure appropriate to the alloy and working after that procedure is precluded. As a result, the manufacturing costs are very high. Using cheaper spool-fed material is impossible.